The present invention relates to calibrating pyrometers that are used in thermal processing systems.
In rapid thermal processing (RTP), a substrate is heated quickly to a high temperature, such as 1200xc2x0 C., to perform a fabrication step such as annealing, cleaning, chemical vapor deposition, oxidation, or nitridation. Particularly given the submicron dimensions of current devices, to obtain high yields and process reliability, the temperature of the substrate must be precisely controlled during these thermal processing steps. For example, to fabricate a dielectric layer 60-80 xc3x85 thick with a uniformity of xc2x12 xc3x85, which is typical of requirements in current device structures, the temperature in successive processing runs cannot vary by more than a few xc2x0 C. from the target temperature. To achieve this level of temperature control, the temperature of the substrate is measured in real time and in situ.
Optical pyrometry is a technology that is used to measure substrate temperatures in RTP systems. An optical pyrometer using an optical probe samples the emitted radiation intensity from the substrate, and computes the temperature of the substrate based on the spectral emissivity of the substrate and the ideal blackbody radiation-temperature relationship.
When the system is first set up, the optical probe must calibrated so that it produces a correct temperature reading when exposed to the radiation coming from the heated substrate. In addition, during repeated use, the temperature sensed by the probe might change over time and thus it will be necessary to recalibrate the probe or at least detect the change that has occurred so that corrective action can be taken. For example, the light pipe which is used to sample the radiation being emitted from the substrate as it is being heated, may become dirty or chipped, connections along the optical column transferring the sampled light to the pyrometer may loosen, or the electronic components in the pyrometer may xe2x80x9cdriftxe2x80x9d.
A commonly used method of calibrating the pyrometer is to use a special substrate or wafer in the chamber. The special substrate, which can be purchased from commercial sources, has a previously measured, known emissivity and it has an xe2x80x9cembeddedxe2x80x9d thermocouple which is attached to the substrate with a ceramic material. When the substrate is heated, its actual temperature is indicated by the thermocouple. Since the substrate""s emissivity is known, the radiation that is actually emitted by the substrate can be easily calculated by multiplying the intensity of radiation that would be expected from by an ideal black body that is at the predetermined temperature times the emissivity of the substrate. This is the radiation level that will be sampled by the optical probe of the pyrometer. The pyrometer is adjusted so that it produces a temperature reading that corresponds to the actual temperature.
Unfortunately, this method has drawbacks. The actual temperature of the substrate may in fact be different than the temperature measured by the thermocouple. First, the presence of the embedded thermocouple and the ceramic material causes the area with the thermocouple to have a different temperature than other parts of the wafer, i.e., it disturbs the temperature profile on the substrate. Second, at high temperatures (e.g., 1000xc2x0 C. as is commonly found in RTP processes) the joint between the wafer and thermocouple tends to degrade, so that after four or five uses the thermocouple readings become unreliable. Because of these shortcomings, this calibration technique cannot really guarantee pyrometer accuracy that is better than ten to fifteen xc2x0 C.
In addition, there are difficulties associated with placing a thermocoupled substrate inside the chamber and making electrical connection to the thermocouple.
In general, in one aspect, the invention features an apparatus for calibrating a temperature probe for a thermal processing chamber. The apparatus features a light source having a stable intensity which is optically coupled to a first end of a fiber optic guide to emit light through a second end of the fiber optic guide during calibration. An alignment mechanism aligns the second end of the fiber optic guide with an input end of the temperature probe. The alignment mechanism includes a first alignment structure to engage a corresponding first alignment feature of the chamber.
Implementations of the invention may include the following features. The fiber optic guide may include a twisted fiber bundle. The light source may be positioned in a cavity in a lighting fixture. The fiber optic guide may be connected to the lighting fixture with the light source positioned to direct light through the first end of fiber optic guide. The alignment mechanism may include a first alignment fixture connected to the second end of the fiber optic guide, and a second alignment fixture, e.g., a disk, having the a plurality of projections adapted to fit a plurality of lift pin holes in a reflector plate. The first alignment fixture may include a second alignment structure, e.g., a step in an outer surface, to engage a corresponding second alignment feature, e.g., a conduit having an annular lip, on the second alignment fixture.
In general, in another aspect, the invention features an apparatus for calibrating a temperature probe that measures a temperature of a substrate during processing within a thermal processing system. The apparatus features a light source optically coupled to a surface to emit light of a predetermined intensity through the surface during calibration. A filter is positioned between the light source and said surface to cause the radiation spectrum emitted from the surface over a predetermined wavelength range to more closely approximate the radiation spectrum of a black body of a predetermined temperature over the predetermined wavelength range. The apparatus also comprises an alignment mechanism for aligning the surface with an input end of the temperature probe.
Implementations of the invention may include the following features. The light source may comprises a light emitting diode (LED). The predetermined wavelength range may be in the infrared, e.g., approximately 0.80 to 0.94 microns. There may be an indicia indicating the predetermined temperature approximated by the surface. The light source may be positioned in a cavity in a lighting fixture. The filter is positioned in the cavity. The surface may comprise an aperture in the lighting fixture or an end of a fiber optic guide.
In general, in another aspect, the invention features a method of calibrating a temperature probe that measures a temperature of a substrate during processing within a thermal processing system. The method features generating light of a stable intensity from a light source, and directing the light to a surface to emit light of a predetermined intensity from the surface during calibration, and aligning the surface with an input end of said temperature probe. The light is filtered by a filter positioned between the light source and the surface to cause the radiation spectrum emitted from the surface over a predetermined wavelength range to more closely approximate the radiation spectrum of a black body of a predetermined temperature over the predetermined wavelength range
Among the advantages of the invention are the following. The pyrometer may be accurately (e.g., less than 1xc2x0 C. error) calibrated without using a wafer with an embedded thermocouple. Calibration may be performed more quickly and using less energy. Calibration may be traced to an absolute standard. The pyrometer may be calibrated without removing the light pipe from the chamber. The calibration instrument may be portable and sturdy.
Other features and advantages will be apparent from the following description and the claims.